Diisobutylaluminum Hydride Promoted Cyclization of 1-Hydrosilyl-4-silyl-1,3-enynes to Polysubstituted Siloles

Article abstract of DOI:10.1021/acs.orglett.7b00038

An efficient method for preparing unsymmetrically multisubstituted siloles is described. The reaction of 1-hydrosilyl-4-silyl-1,3-enynes with diisobutylaluminum hydride (DIBAL-H) gave multisubstituted siloles in good to high yields. This method could be applied to the synthesis of benzosiloles using 2-hydrosilyl-1-(silylethynyl)benzenes as substrates. The silole formation was also promoted even by a substoichiometric amount of DIBAL-H. The reaction provides a straightforward method to prepare siloles and benzosiloles.

Full text of DOI:10.1021/acs.orglett.7b00038

Letter
pubs.acs.org/OrgLett
Diisobutylaluminum Hydride Promoted Cyclization of 1‑Hydrosilyl-4-
silyl-1,3-enynes to Polysubstituted Siloles
Hidenori Kinoshita, Akihiro Ueda, Hiroki Fukumoto, and Katsukiyo Miura*
Department of Applied Chemistry, Graduate School of Science and Engineering, Saitama University, 255 Shimo-ohkubo, Sakura-ku,
Saitama 338-8570, Japan
*
S Supporting Information
ABSTRACT: An efficient method for preparing unsymmetri-
cally multisubstituted siloles is described. The reaction of 1-
hydrosilyl-4-silyl-1,3-enynes with diisobutylaluminum hydride
(
DIBAL-H) gave multisubstituted siloles in good to high yields.
This method could be applied to the synthesis of benzosiloles
using 2-hydrosilyl-1-(silylethynyl)benzenes as substrates. The
silole formation was also promoted even by a substoichiometric
amount of DIBAL-H. The reaction provides a straightforward method to prepare siloles and benzosiloles.
7
,8
iloles have unique electronic and photophysical properties
due to the low energy of the lowest unoccupied molecular
silylene equivalents (eqs 4 and 5). Despite all of the efforts
that have been made toward the efficient synthesis of siloles,
the regiodefined synthesis of unsymmetrically substituted
siloles bearing different substituents has largely remained
unexplored. There is much room for developing new synthetic
methods to extend the diversity of the potentially beneficial
compounds. To address this issue, we adopted a preinstallation
approach via intramolecular C−Si bond formation of linear
substrates bearing desired substituents at the defined positions.
We report herein that the DIBAL-H-promoted cyclization of 1-
hydrosilyl-4-silyl-1,3-enynes 1 and related compounds 3 is
valuable for the regiodefined synthesis of unsymmetrically
substituted siloles (Scheme 2).
S
1
,2
orbitals compared with their carbon analogues. Therefore,
much attention has been focused on their use as electron-
transport and light-emitting materials for organic electron
3
devices. Synthetic methods for substituted siloles have also
been investigated intensively to impart more efficient properties
by variations of the substituents. The conventional methods for
silole synthesis are those via reductive cyclization, which
provide rapid access to symmetrically substituted siloles by a
4
,5
successive bond-forming process (eqs 1 and 2 in Scheme 1).
Excellent approaches to substituted siloles have also been
achieved by transition-metal catalysis as shown in the Ru-
6
catalyzed double trans-hydrosilylation of 1,3-diynes (eq 3) and
the Ni- and Pd-catalyzed [2 + 2 + 1] cycloadditions using
Scheme 2. DIBAL-H-Promoted Silole Synthesis
Scheme 1. Known Methods for Silole Synthesis
9
In the course of our study on synthetic uses of DIBAL-H,
we found that the treatment of 1,3-enyne 1a, bearing a
hydrosilane moiety, with DIBAL-H gave silole 2a (Table 1).
With this finding, our goal was set to develop a new method for
silole synthesis by the DIBAL-H-promoted cyclization. We
commenced the optimization of reaction conditions using 1a,
which can be accessed easily from known 1-bromo-1,3-
10
enynes, as a probe. The reaction of 1a with 1.5 equiv of
DIBAL-H at 75 °C for 24 h afforded 2a in 64% yield (entry 1).
When the isolated product 2a was treated with DIBAL-H (1.5
equiv) under the same conditions, it was converted into a
complex mixture of products. This observation implied that 2a
Received: January 5, 2017
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b00038
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 1. Optimization of Reaction Conditions
b
entry
DIBAL-H (equiv)
time (h)
temp (°C)
yield of 2a (%)
1
2
3
4
5
6
7
8
9
1.5
1.5
1.5
1.5
1.5
1.2
1.0
0.5
0.5
0.5
0.3
0.3
0.3
0.3
0.2
0.1
1.2
1.2
1.2
24
2
75
75
75
75
75
75
75
75
75
80
75
75
75
80
75
75
70
80
90
64
76
86
92
1
0.5
0.17
0.5
0.5
0.5
1
c
c
,
d
81 (11)
93
91
c
c,d
83 (4)
81
1
1
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
8
9
3
81
c
c
c
c
c
,
,
,
,
d
d
d
d
1
40 (48)
c
2
63 (20)
c
4
69 (13)
c
2
64 (19)
e
2
0
e
2
0
c
c,d
0.5
0.5
0.5
76 (9)
96
84
Figure 1. Isolated yields of siloles 2. Conditions: All reactions were
carried out with 1 (0.25 mmol) and DIBAL-H (0.30 mmol) in octane
a
Unless otherwise noted, all reactions were carried out with 1a (0.25
(0.75 mL) at 80 °C for 0.5 h. (a) A substoichiometic amount of
b
c
mmol) in octane (0.75 mL). Isolated yield. NMR yield (Bn O as an
internal standard). Recovery of 1a in parentheses. 1a was recovered
almost quantitatively.
2
DIBAL-H (0.125 mmol) was used. The reaction time was 1 h. (b) The
reaction time was 24 h.
d
e
silicon, was obtained in excellent yield. The cyclization to siloles
3
4
3
4
3
4
underwent DIBAL-H-promoted side reactions during the
reaction of 1a. This is the reason that the reaction time was
scrutinized (entries 2−5). The time variations revealed that the
reaction was completed within 0.5 h and that 2a was gradually
decomposed as predicted. The reaction for 0.5 h achieved a
2g (R = R = Et), 2h (R = R = n-C H ), 2i (R = R =
5 11
3
4
CH SiMe ), and 2j (R -R = (CH ) ) also proceeded in good
2
3
2 4
to excellent yields. Siloles 2k and 2l, bearing four different
substituents at the four ring atoms, could also be prepared from
the corresponding enynes 1k and 1l. Even with 0.5 equiv of
DIBAL-H, the reactions of the selected substrates 1c, 1f, 1i, and
1k gave the corresponding siloles 2c, 2f, 2i, and 2k,
respectively, in satisfactory yields.
When enyne 1m, bearing no substituent at the alkyne
terminus, was subjected to the standard reaction conditions,
only the reduced product 2m′ was formed in 43% yield
together with the recovery of 1m (Scheme 3). Nonsilylated
92% isolated yield of 2a (entry 4). Then the effect of amount of
DIBAL-H was investigated (entries 6−16). The cyclization
using 1.2 equiv of DIBAL-H proceeded in 93% yield (entry 6).
It is noteworthy that a substoichiometric amount of DIBAL-H
(
8
0.5 equiv) was enough to promote the reaction (entries
−10), although an elongated reaction time was needed to
complete the conversion of 1a. With less than 0.5 equiv of
DIBAL-H, the reaction was not completed (entries 11−16). In
particular, the reaction using 0.1 or 0.2 equiv of DIBAL-H did
not give 2a at all, and an almost quantitative yield of 1a was
recovered. Finally, variations in reaction temperature were
examined in the reaction with 1.2 equiv of DIBAL-H for 0.5 h
Scheme 3. Reactivity of Enynes 1m and 1n
(
entries 17−19). As a result, 2a was obtained in 96% isolated
yield at 80 °C (entry 18).
With the optimized reaction conditions in hand, the scope of
this transformation was investigated (Figure 1). Enynes 1b and
1
c, bearing a different silyl group at the alkyne terminus, were
submitted to the optimized conditions. The desired siloles 2b
and 2c were formed in excellent yields. The larger scale reaction
of 1b (1 mmol) gave 2b in 83% isolated yield. Enyne 1d,
bearing a dimethylsilyl group at the alkene terminus, was
cyclized to 2d in 70% yield. The lower yield is due to
unidentified side reactions of 1d under the reaction conditions.
Introduction of a diisopropylsilyl group into the substrate at the
alkene terminus resulted in a slow cyclization leading to 2e.
Elongation of the reaction time improved the yield of 2e. Silole
internal alkyne 1n was inert under the same conditions and was
recovered quantitatively. These results indicate that the silyl
group at the alkyne terminus (SiR5 in 1) is crucial for the
3
present transformation.
We next examined the application of the DIBAL-H-
promoted cyclization to benzosilole synthesis (Figure 2).
1
2
2
f, bearing different substituents (R = Me, R = Ph) at the ring
B
DOI: 10.1021/acs.orglett.7b00038
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Organic Letters
Letter
Scheme 4. Plausible Mechanism for Formation of Siloles 2
essential role of the alkynylsilane moiety for the present
cyclization (Scheme 3) agrees with the proposed mechanism.
Unlike the reaction of 1e, the formation of alkenylalane (E)-2a′
from 1a was not detected even at the time when 1a was not
fully converted (Table 1, entry 5). This difference between the
reactions of 1a and 1e indicates that the silicon−carbon bond-
forming step with 1a is much faster than that with 1e and that
the reaction rate is sensitive to steric congestion around the
silicon atom. Therefore, it is likely that the intramolecular
reaction of alkenylalanes B proceeds by nucleophilic attack of
Figure 2. Isolated yields of benzosiloles 4. Conditions: All reactions
were carried out with 3 (0.25 mmol) and DIBAL-H (0.30 mmol) in
octane (0.75 mL) at 80 °C for 0.5 h. (a) A substoichiometic amount of
DIBAL-H (0.125 mmol) was used. The reaction time was 1 h.
There are several intriguing methods for the synthesis of
6
,11−13
benzosiloles from hydrosilanes.
However, to the best of
our knowledge, use of DIBAL-H for this purpose has been
unknown so far. The treatment of 1-hydrosilyl-2-(silylethynyl)-
benzenes 3a with DIBAL-H gave the desired benzosilole 4a in
15
78% yield under the standard conditions. The cyclization to
the α-carbon to the silicon center.
other benzosiloles 4b−f successfully proceeded in excellent
yields. Similar to the case of 1, a substoichiometric amount of
DIBAL-H was enough for efficient cyclization of 3d and 3e.
As shown above (Figure 1), the cyclization of enyne 1e
required a prolonged reaction time. The reaction for 30 min
resulted in 76% conversion of 1e to give the reduced,
uncyclized product (E)-2e′ as the major product as well as
the desired product 2e (Table 2, entry 1). The Z-isomer of 2e′
The efficient cyclization with a substoichiometric amount of
DIBAL-H implied the regeneration of DIBAL-H in the silicon−
carbon bond-forming reaction of B. To prove this hypothesis, a
deuterium-labeling experiment was conducted as shown in
Scheme 5. The reaction of deuterated enyne 1b-d with 0.5
Scheme 5. Deuterium-Labeling Experiment
a
Table 2. Time Variations of Reaction of Enyne 1e
equiv of DIBAL-H at 80 °C for 1 h gave nondeuterated silole
b
2
b and 3-deuterated silole 2b-d in 34% and 51% yields,
yield (%)
respectively. On the basis of the proposed mechanism in
Scheme 4, this result can be reasonably explained by the
following series of events (Scheme 6). The first reaction
sequence promoted by DIBAL-H proceeds via alkenylalanes C
entry
time (h)
1e
(E)-2e′
2e
1
2
3
0.5
1
24
7
26
11
0
21
43
c
24
0
63
and D to form 2b and DIBAL-D [(i-Bu) AlD]. The second or
2
a
The reactions were carried out with 1e (0.25 mmol) and DIBAL-H
subsequent reaction sequence (cycle) is promoted by the
regeneratable DIBAL-D to give 2b-d via alkenylalanes E and F.
From this consideration, the formation of 2b-d is indicative of
the generation of DIBAL-D from D. Thus, the result of the
deuterium-labeling experiment supports the regeneration of
DIBAL-H from B (Scheme 4). Although the origin of the low
catalytic turnover number of DIBAL-H is not clear, the
deactivation of DIBAL-H is possibly due to thermal
b
(
0.30 mmol) in octane (0.75 mL) at 80 °C. NMR yield (Bn O was
2
c
used as an internal standard). Isolated yield.
was not detected in the reaction mixture. The conversion at 1 h
reached 93% (entry 2). At this reaction time, 2e became the
major product with a decrease in the yield of (E)-2e′. After 24
h, both 1e and (E)-2e′ were completely consumed, and only
silole 2e could be identified as a product (entry 3).
decomposition and the reaction with adventitious air (H O
2
The capture of (E)-2e′ and the time variations of the
reaction of 1e suggest that the reaction mechanism involves
hydroalumination of the alkynylsilane moiety of 1 with DIBAL-
H, geometrical isomerization of the initially formed alkenyla-
and O ) and the siloles formed.
2
In summary, we have developed an efficient method for
preparing unsymmetrically substituted siloles from 1-hydrosilyl-
4-silyl-1,3-enynes and DIBAL-H with complete regioselectivity.
This method could be successfully applied to the synthesis of
benzosiloles. These reactions successfully proceeded even with
a substoichiometric amount of DIBAL-H. The present study
has disclosed a novel example of silicon−carbon bond
9
,14
lanes A to B, and subsequent intramolecular silicon−carbon
bond formation (Scheme 4). Since triorganosilyl groups at the
sp-carbon are known to promote alkyne hydroalumination and
9
,14
geometrical isomerization of the resulting alkenylalanes,
the
C
DOI: 10.1021/acs.orglett.7b00038
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Organic Letters
Letter
1
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Scheme 6. Plausible Mechanism for Formation of Silole 2b-d
1
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15) For nucleophilic substitution of hydrosilanes with carbon
ASSOCIATED CONTENT
Supporting Information
■
4
(
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b00038.
nucleophiles at silicon, see: Corriu, R. J. P.; Guerin, C.; Moreau, J. J. E.
In The Chemistry of Organic Silicon Compounds; Patai, S., Rappoport,
Z., Eds.; Wiley: Chichester, 1989; Vol. 1, Chapter 4, p 305.
Experimental procedures and spectroscopic data for all
new compounds (PDF)
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: kmiura@apc.saitama-u.ac.jp.
ORCID
Hidenori Kinoshita: 0000-0002-9761-9015
Katsukiyo Miura: 0000-0002-4535-8677
Notes
The authors declare no competing financial interest.
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DOI: 10.1021/acs.orglett.7b00038
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